1. Field of the Invention
The present invention relates to packet switching techniques, and in particular to packet switching system and method ensuring traffic quality.
2. Description of the Related Art
LANs (local-area networks) and IP (Internet Protocol)-based private networks have been widely used in companies and universities and are undergoing further development to carrier-class public networks. As the demands for telephony traffic and online trading transactions are growing, real-time and reliable traffic transfer becomes more important. In such a background, there have been proposed packet switching techniques allowing Quality of Service (QoS) guarantees to be maintained.
An ATM (asynchronous transfer mode) cell switching system guaranteeing the sequence and continuity of cells has been disclosed in Japanese Patent Application Unexamined Publication No. 5-7213. More specifically, the conventional switching system is provided with working and reserved ATM cell switches, which are selectively connected to an outgoing line by a system selector. When the working ATM cell switch is switched to the reserved ATM cell switch, the system switching is performed after all the cells staying in the working ATM cell switch have been completely forwarded to the outgoing line. This cell switching technique can be also applied to IP packet switching systems.
In the case of traffic flows requiring real time with little delay, it is necessary to pass the packets through the switch in relatively short time. On the other hand, some traffic flows that do not necessitate real time may be permitted to be transferred with relatively long delay.
According to the above-described prior art (Japanese Patent Application Unexamined Publication No. 5-7213), however, the system switching from the working ATM cell switch to the reserved ATM cell switch is performed after all the cells abiding in the working ATM cell switch have been completely forwarded to the outgoing line. Therefore, traffic requiring real time is kept waiting in the reserved ATM cell switch until the working ATM cell switch have completely forwarded the abiding cells to the outgoing line. This may not ensure required QoS.
In general, in the case of real-time traffic, an end terminal is provided with a buffer for absorbing variations in arrival time of packets. However, it is necessary for packets to arrive within a predetermined delay time. For example, telephone conversation cannot be don smoothly without limiting a delay time to at most several hundred milliseconds. If a packet is delayed by a time interval longer than an absorbable time period, then the packet is assumed not to arrive and is interpolated, or equivalently loss of packet.